This invention relates to an improved expansion anchor bolt assembly and, in particular, to an expansion anchor bolt assembly utilizing a segmented expansion shell that generally exhibits the holding characteristics of a more costly single piece unit.
Anchor bolt assemblies of this type are used in a wide range of applications. The most important application, however, is the area of mine safety where the expansion device is employed to help support the roof of the mine. Typically, a hole is drilled or bored upwardly through the ceiling of the mine shaft into the overhead stratum. An elongated bolt, containing a plate at the head end and an expansion anchor assembly at the thread end, is passed upwardly into the opening. A wedge, contained in the expansion anchor assembly, is drawn down into the anchor assembly by turning the bolt. This, in turn, forces a number of finger-like elements to be expanded outwardly into holding contact against the wall of the opening and also draws the plate against the mine shaft roof. Further torquing of the bolt stresses the overhead formation of earth and rock to prevent it from moving and thus creating a potentially dangerous situation.
Heretofore, it had been preferred to construct the expansion shell of the anchor from a single piece of generally malleable metal. In form, the shell conventionally includes an annular base section having a plurality of finger-like elements axially extended from the top surface thereof. The single piece base provides a foundation of considerable strength about which the relatively weaker fingrs deform as the wedge is drawn into the anchor assembly. As a result, the fingers move uniformly in an outward direction to provide for a parallel expansion of the shell members. It is well known in the art that this type of expansion delivers the greatest possible holding power and anything less will reduce the stressing ability of the assembly. The main disadvantage of the single piece shell relates to the high cost of producing this relatively intricate structure and the excessive scrap rate associated therewith.
Attempts have been made, with varying degrees of success, to fabricate expansion shells in two or more parts which can be more conveniently and economically formed by well known casting, stamping and/or forging techniques. Bringing the parts together in assembly and properly supporting the parts during the expansion process has long been a problem in the art.
A two or four leaf bail assembly is probably the most common type of segmented expansion shell device that is used in industry today. In this arrangement, two shell segments are generally joined together by a bail or strap by staking or welding the strap to the top section of each segment. In operation, a draw bolt acts between the strap and a wedge to force the wedge downwardly against the shell fingers. The shell is permitted to open at the bottom with each segment being deformed outwardly about the relatively thin strap. Any small resistance to the movement of one or both segments is immediately translated to the weaker strap, causing it to twist and/or bend in an unpredictable manner. As a result, parallel expansion of the shell segments cannot be maintained and less than optimum anchoring is delivered.
Another common anchor arrangement involves joining two or more shell segments at the base of the shell by means of a clip-on "pal nut" that is formed from a thin sheet metal stamping. A threaded opening is provided in the stamping through which the draw bolt is turned into threaded engagement with the wedge. In practice, the bolt acts between the two threaded members to pull the wedge down into deforming contact against the shell fingers. Here again, the clip-on nut represents the weak element in the system and, as the wedge descends, the clip will usually release prematurely thereby allowing the shell components to be forced out of their original optimum operational position. As a consequence less than maximum performance is produced by the assembly. A clip-on device of the type noted is described in further detail in U.S. Pat. No. 3,941,028.
Both the bail and clip-on nut mechanisms perform quite well in holding the expansion device together in assembly. They also provide a convenient means for allowing the wedge to move into operation against the fingers. However, in both arrangements, the retaining element forms a weak section in the assembly which usually deforms prematurely under stress thereby causing the anchor to fail or deliver less than the desired holding power.